The general aim of the proposed study is to examine the relation between linguistic and cognitive development. The proposed research focuses on how children's increasing knowledge of a complex event domain (i.e., one involving actors, actions and props and their interrelations) both structures and supports their language acquisition. The study has three specific objectives: (1) To determine how increasing knowledge of a domain affects children's language development and use; (2) To determine how mothers structure and support this process; and (3) To determine the effect of mothers' language in event contexts in which children have different levels of expertise ON THEIR CHILDREN'S LATER LANGUAGE DEVELOPMENT. Previous studies have argued for the importance of shared event knowledge between mothers and children, but have not provided strong empirical evidence for such a role. The proposed study seeks to evaluate the effect of event knowledge on language acquisition, but attempts to assess its effects on different components of linguistic development and assess its long term effects on language acquisition. To assess these objectives children at THREE age groups (18, 24, & 30 months of age) will be followed for 10-WEEKS, interacting with their mothers in two event contexts. These events contexts are comprised of complex toys (e.g., airports, farms, etc.) in which there are multiple components and interrelations between the components. In the expert-context, the mother-child dyads interact with the same toy during each observation period. In effect the children will become "experts" in that event domain over time. In the novice-context, these same dyads also will be observed interacting with a different novel toy during each observation session. In this novel-context, the children will not be able to become knowledgeable about the event. Of interest is the comparison between these event contexts in both children's and mother's language use. Furthermore, the study will also evaluate the relation between mothers' language in these different contexts on children's language development SIX MONTHS LATER. Of interest is whether maternal language in the expert-event context is a better predictor of later language development than maternal language in the novice-event context.